1. Field of the Invention
The invention relates to a retort furnace for the heat treatment, for example, the blank annealing, annealing under nitrogen or nitrogen/hydrogen, nitriding, or nitrocarburizing of metal workpieces.
2. Description of the Related Art
Retort furnaces for the heat treatment of metal workpieces are known in manifold configurations according to the prior art, for example, according to DE-AS 2 010 433, DE-OS 27 54 034, DE 30 28 952 C2, DE 31 43 532 A1, DE 36 31 389 C2, and DE 103 38 431 A1.
The invention does prefer horizontal retort furnaces, which essentially have a recumbent tubular retort, a furnace housing which encloses the retort with thermal insulation, and a unit for heating the retort, but it is also transferable to other constructions of retort furnaces.
The retort typically has                a treatment or batch space, which receives the protective and reactive gases and is closable gastight, for the heat treatment of the workpieces as batches and        appropriate frameworks for the expedient positioning and receiving/support of the batches.        
During the heat treatment, the heating to temperatures up to 650° C. is substantially supported by convection. An effort is made for an intensive circulation of the furnace atmosphere which encompasses the workpieces on all sides within the retort. For this purpose, circulation assemblies and gas guiding aids situated in the area of the retort are used.
In particular during the heat treatment, the retorts are subjected to high strain. They are implemented having wall thicknesses greater than 8 mm because of the dimensional stability.
At least in the retort furnaces of the above-described type, in contrast, spot-shaped stresses result in the lower part of the retorts, which are horizontal in this case, due to the batch weights and the support elements of the receptacle frameworks. It is disadvantageous that the stresses rise with increasing batch weight, while in contrast the carrying capacity of the retort material decreases with increasing furnace temperature. Therefore, the loading capacity and the efficiency of the heat treatment of the workpiece batches of such retort furnaces are limited.
The technical world has already concerned itself with preventing harmful strains of retorts during the heat treatment of metal workpieces. However, such solutions only relate to the relief of the retort floor in the case of vertically situated retorts by supports and/or by load-carrying means for support in relation to a housing as described, for example, in DE 2 054 666 A.
The strain of the retorts due to the batch weights themselves and their receptacle frameworks was taken into consideration, but the efficiency of the heat treatment was not increased.
Furthermore, a compensator technology (IHU) was known from the exclusively vertical furnace construction, in which the entire batch load rests on a heat resistant pedestal made of concrete and is supported thereon. The retort, whose impact is compensated for via a compensator, can thus be implemented having a thinner wall thickness, and its service life lengthens because of the reduced load.
However, in addition to undeniable advantages, disadvantages also result from this solution, such as:                The solution is not transferable to horizontal configurations.        The pedestal and further ceramic insulation materials and supports may bind moisture which is harmful for subsequent processes.        The compensator is relatively large and costly because of its construction; it forms heat bridges and causes energy losses.        The configuration in the floor encourages the transition to lower temperatures and the formation of condensate, which is more difficult than water vapor to remove from the retort.        
In addition, a heat treatment furnace, in particular a pressure sintering furnace having a furnace housing and an insulation cage, which is held by the furnace housing and forms a boiler room and encloses it at a distance, and a muffle, which is situated in the interior of the insulation cage, envelops the workpiece batch, and delimits an insulation space with the insulation cage, was already described in DE 103 12 650 B3. It also comprises so-called supports as a type of receptacle framework for the batch. Transferring this solution to retort furnaces according to the species of the type described at the beginning would not result in increasing the efficiency of the heat treatment of the workpiece batches and the loading capacity of the retorts.